<!doctype html>
<html lang="en">
<head>
	<meta charset="UTF-8" />
	<title>橡皮擦效果</title>
	<style type="text/css">
		.box{
			position: absolute;
			left:0;
			top:0;
			width: 100%;
			height: 100%;
			background:#000 url("img/1.png") no-repeat center center;
			background-size: auto 100%;
			backface-visibility: hidden;
			overflow: hidden;
		}
	</style>
</head>
<body>
	<div class="box" id="bb">
		<canvas id="cas" ></canvas>
	</div>

<script src="js/index.js"></script>
</body>
</html>